1. Field of the Invention
This invention generally relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a reduced volume method and apparatus for sampling and testing a formation fluid using multiple regression analysis.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. A large portion of the current drilling activity involves directional drilling, i.e., drilling boreholes deviated from vertical and/or horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from earth formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
One type of while-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well, reducing a test volume pressure, and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different reservoirs within a given borehole or at several points in a single reservoir. This type of test is known as a xe2x80x9cPressure Build-up Test.xe2x80x9d One important aspect of data collected during such a Pressure Build-up Test is the pressure build-up information gathered after drawing down the pressure in the test volume. From this data, information can be derived as to permeability and size of the reservoir. Moreover, actual samples of the reservoir fluid can be obtained and tested to gather Pressure-Volume-Temperature data relevant to the reservoir""s hydrocarbon distribution.
Some systems require retrieval of the drill string from the borehole to perform pressure testing. The drill string is removed, and a pressure measuring tool is run into the borehole using a wireline tool having packers for isolating the reservoir. Although wireline conveyed tools are capable of testing a reservoir, it is difficult to convey a wireline tool in a deviated borehole.
The amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, when a hole is highly deviated wireline conveyed test figures cannot be used because frictional force between the test rig and the wellbore exceed gravitational force causing the test rig to stop before reaching the desired formation.
A more recent system is disclosed in U.S. Pat. No. 5,803,186 to Berger et al. The ""186 patent provides a MWD system that includes use of pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. The ""186 device enables obtaining static pressures, pressure build-ups, and pressure draw-downs with a work string, such as a drill string, in place. Also, computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without removing the drill string from the borehole.
Using a device as described in the ""186 patent, density of the drilling fluid is calculated during drilling to adjust drilling efficiency while maintaining safety. The density calculation is based upon the desired relationship between the weight of the drilling mud column and the predicted downhole pressures to be encountered. After a test is taken a new prediction is made, the mud density is adjusted as required and the bit advances until another test is taken.
A drawback of this type of tool is encountered when different formations are penetrated during drilling. The pressure can change significantly from one formation to the next and in short distances due to different formation compositions. If formation pressure is lower than expected, the pressure from the mud column may cause unnecessary damage to the formation. If the formation pressure is higher than expected, a pressure kick could result. Consequently, delay in providing measured pressure information to the operator may result in drilling mud being maintained at too high or too low a density.
Another drawback of the ""186 patent, as well as other systems requiring large fluid intake, is that system clogging caused by debris in the fluid can seriously impede drilling operations. When drawing fluid into the system, cuttings from the drill bit or other rocks being carried by the fluid may enter the system. The ""186 patent discloses a series of conduit paths and valves through which the fluid must travel. It is possible for debris to clog the system at any valve location, at a conduit bend or at any location where conduit size changes. If the system is clogged, the tool must be retrieved from the borehole for cleaning causing delay in the drilling operation. Therefore, it is desirable to have an apparatus with reduced risk of clogging.
Another drawback of the ""186 patent is that it has a large system volume. Filling a system with fluid takes time, so a system with a large internal volume requires more time for the system to respond during a drawdown cycle. Therefore it is desirable to have a small internal system volume in order to reduce sampling and test time.
The present invention addresses some of the drawbacks discussed above by providing a measurement while drilling apparatus and method which enables sampling and measurements of parameters of fluids contained in a borehole while reducing the time required for taking such samples and measurements and reducing the risk of system clogging.
One aspect of the present invention provides a method for determining a parameter of interest of a formation while drilling. The method comprises conveying a tool on a drill string into a borehole traversing the formation and extending at least one selectively extendable probe disposed on the tool to make sealing engagement with a portion of the formation. A port is exposed to the sealed portion of the formation, the port providing fluid communication between the formation and a first volume within the tool. The first volume is varied with a volume control device using a plurality of volume change rates. The method includes determining at least one characteristic of the first volume using a test device at least twice during each of the plurality of volume change rates, and using multiple regression analysis to determine the formation parameter of interest using the at least one characteristic determined during the plurality of volume change rates.
Another aspect of the present invention provides a method for determining a parameter of interest of a formation while drilling. The method comprises conveying a tool on a drill string into a borehole traversing the formation and extending at least one selectively extendable probe disposed on the tool to make sealing engagement with a portion of the formation. A port is exposed to the sealed portion of the formation, the port providing fluid communication between the formation and a first volume within the tool, the first volume being selectively variable between zero cubic centimeters and 1000 cubic centimeters. The first volume is varied with a volume control device using a plurality of volume change rates. The method includes determining at least one characteristic of the first volume using a test device at least twice during each of the plurality of volume change rates, and determining the formation parameter of interest using the at least one sensed characteristic sensed during the plurality of volume change rates.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts.